A sufficiently fast evolution can easily be called a revolution. The industrial revolution was actually just an evolution of industry, yet everyone does know it as a revolution. Why? Because it happened really fast.
The title does say it *could* revolutionize. It may just be a small improvement, or fail completely, but it could be a revolution if it suddenly brings fast, cheap, high-density memory in a scale much greater than Flash memory is able to provide.
The only hardware-accelerated operations of the GDI are color filling, blitting, alpha-blending, and stretching (with or without alpha). Everything related to drawing shapes (polygons, curves,
If anything feels snappier in older OSes, it would be because before XP, Microsoft still supported old hardware blitters, such as what was used by DirectDraw. Support for hardware blitting was lost with the switch away from the Win9X architecture and embracing NT. I may be remembering wrong on this, but I believe XP was missing the faster blitting, but didn't support the same level of hardware-acceleration as newer versions of Windows, so in theory, it should have been the least snappy of all...
As to the % of applications using GDI, I'd make it more like >95%, and I'm being conservative. The only application I know for certain uses Direct2D or Direct3D for the interface drawing would be Visual Studio 2010 or newer. And XUL-based applications such as Firefox, where d2d/d3d is supported and available. As far as I know even Java applications which custom-draw the GUI end up doing so with GDI functions.
XP was complete bloat by 2001's standards. Windows 7 was made lighter than Vista, after people complained about Vista the same way people complained about XP.
The only reason XP doesn't feel bloated right now, is because it's 13 years old and 10 years obsolete -- since it can't do a lot of things OSes are expected to do (not by you obviously). The biggest mistake Microsoft made wasn't Vista's expectation of a decent computer, nor windows 8's oversimplification. It was letting people grow used to XP for way too long.